Such energy-saving and energy -recovery devices, as disclosed in PCT/WO 93/11363 and DE 44 38 899 C1, use traditional hydraulic-pneumatic accumulators as energy reservoirs. In the energy recovery device disclosed in the PCT specification, the piston chamber of a hydraulically operable working cylinder is connected with the hydraulic accumulator through a cartridge valve. The cartridge valve cooperates with a control manifold connected to a pressure relay as part of a fluid control system. This fluid control system arrangement is in turn connected at its control input to a low-pressure branch of the hydraulic circuit, which cooperates with the displaceable parts of the working machinery in the form of active working assemblies or the like. With lowering of the active working assembly or the like, the fluid volume on the piston side of the working cylinder is integrated with the relevant available potential energy. The fluid is then discharged under pressure to the hydraulic reservoir/accumulator, and can be recycled from there precisely quantifiably for the subsequent lifting or raising process concerning the active working assembly to recover the energy introduced into the hydraulic accumulator. A good energy recovery ratio is attained with this arrangement, insofar as three or more working cylinders are used as hydraulically working active working assemblies. For all practical purposes three or more working cylinders are not used, especially with hydraulically operable machinery such as steam shovels or excavators or the like. Also, while holding the active working assemblies, the cartridge valves are inclined to be lowered under the effect of the load until they oscillate or flutter. The oscillating or fluttering leads to an undesired rocking of the active working assembly arrangement, usually in the form of a steam shovel, excavator or crane jib.
In DE 44 38 899 C1 such cartridge valves are abandoned in favor of hydraulic lockable check valves in a connecting conduit extending between the hydraulic accumulator and the hydraulic machinery to be operated. This arrangement is also of reasonable cost and functionally reliable. However, it has been shown in practice that with hydraulic operable working cylinders, upon discharge of the recovered energy with the associated volume of fluid, these cylinders are strongly influenced in a negative sense, which can lead to stoppages in the train of movement.
An operating device for a hydraulically operable working cylinder is disclosed in U.S. Pat. No. 2,721,446. Continuous operation maintenance for hydraulically operated cylinder is obtained through a hydraulic pump protected by a check valve. When interference and interruption occur in the sense of need for emergency/temporary supply, the piston accumulator using two longitudinally displaced pistons and working through the preloaded internal gas pressure in the preloaded chamber secures further hydraulic supply for the working cylinder and causes displacement of the same. The ambient atmosphere chamber of the known piston accumulator arranged at the opposite end of the preloaded chamber extends through a ventilation opening into the ambient atmosphere. A supply line is guided in the branch between the piston accumulator and the working cylinder, protected by means of the check valve in relation to the hydraulic pump, and allows for the resulting emergency/temporary supply to the working cylinder. The supply line does not allow for an erroneous hydraulic connection, such as would be possible with this known device, for a continuous energy savings during operation of the working cylinder. Thus, an undesirable temperature rise caused by compression in the air-filled ambient atmosphere chamber results and the fluid volumes to be controlled for emergency/temporary operation turn out to be correspondingly large, which is unfavorable from the point of view of savings of energy.